In the prior art, it is known to provide, as a means for measuring the concentration of an optically active substance in a sample, a method that obtains the concentration from the optical rotation that a beam of light experiences when it is passed through the sample. On the other hand, enzyme-based methods such as the GOP method are known, for example, for measuring glucose concentrations. In the GOP method, there is a limit to the number of measurements that can be made, because an electrode must be brought into contact with the sample and because of its principle of measurement; as a result, maintenance which involves replacing part of the measurement apparatus, adding a buffer solution, etc. must be performed periodically.
In contrast, in the optical rotation measuring method that uses a beam of light, as the measurement can be made without contacting the sample directly, the measurement apparatus can be used, without requiring any particular maintenance work, for a relatively long period of time. The period depends on the life of the light source, the degree of contamination of the sample cell, etc.
The optical method based on the optical rotation, etc. has the further advantage that there is no danger of a human subject accidentally touching the sample such as urine and the measurement can be done without making the subject particularly conscious of it.
The principle of the method that obtains the concentration of an optically active substance in a sample from the optical rotation produced by it is based on the following equation (1).θ(λ)=α(λ)·c·L  (1)where θ(λ) is the optical rotation when the wavelength of the light beam is denoted by λ, α(λ) is the specific rotation of the optically active substance when the wavelength of the light beam is denoted by λ, c is the concentration of the optically active substance in the sample, and L is the optical path length through the sample. In the equation 1, the specific rotation α(λ) is a coefficient unique to the optically active substance (though it varies depending on the wavelength λ of the light beam or on temperature) and is therefore a value known before the measurement of the concentration. The optical path length L through the sample is also a value known before the measurement of the concentration. Accordingly, the concentration, c, of the optically active substance can be determined by measuring the optical rotation θ(λ) when the beam is passed through the sample.
Here, the optical rotation is obtained in the following manner: first, linearly polarized light is directed into the sample, and the light passed through the sample is input to an analyzer; then, the light passed through the analyzer is input to a photodetector such as a photodiode for conversion into an electrical signal, from which the optical rotation is obtained.
That is, when the tilt angle of the transmission axis of the analyzer with respect to the transmission axis of the polarizer is denoted by φ, and the optical rotation produced by the sample is denoted by β, then the intensity, I, of the light received by the photodetector can be determined based on the following equation (2).I=T×I0 cos2(φ−β)  (2)where T represents the transmittance considering all of the attenuations due to reflections and absorptions occurring in the sample, the polarizer, and the analyzer, and I0 designates the intensity of the incident light. As can be understood from the equation (2), the intensity of light, I, changes as φ changes, and a minimum point is obtained for every rotation angle π(rad). Accordingly, the optical rotation β produced by the sample can be obtained by measuring the intensity of light, I, when φ is changed.
One possible method to change φ would be to rotate the polarizer or the analyzer. However, the method of rotating the polarizer or the analyzer has had the problem that the apparatus size becomes relatively large, because it requires mechanical manipulation to rotate. In view of this, there is proposed a method that electrically modulates the plane of polarized light by using a Faraday element (for example, refer to patent document 1) or a liquid crystal element as a polarization rotator.
A Senarmont polarization rotator constructed by combining a liquid crystal element with a quarter-wave plate is an example that uses the liquid crystal element to rotate linearly polarized light. There is also proposed an apparatus in which three liquid crystal elements capable of being supplied with variable voltage are arranged in series along the direction of propagation of light, thereby achieving light modulation with greater freedom (for example, refer to patent document 2). Further, there is proposed an optical measurement apparatus that utilizes the optical characteristics of a liquid crystal element, eliminating the need for conventional mechanical moving parts (for example, refer to patent document 4). There is also proposed an apparatus that achieves highly accurate and stable measurements by periodically performing phase modulation using a liquid crystal element (for example, refer to patent document 3).
Further, when the sample is urine, a method is disclosed that detects a urine component by measuring its optical rotation (for example, refer to patent document 1).
FIG. 25 shows the optical system implementing the above method.
In FIG. 25, a beam of light emitted from a light source 21 such as a laser diode is collimated by a collimating lens 22 into a parallel beam of light, which is converted by a polarizer 23 into linearly polarized light vibrating in a vertical direction. The linearly polarized light passed through the polarizer 23 enters a liquid crystal element 31 where the polarization component in the direction of +45 degrees or −45 degrees relative to the vertical is phase-modulated. In the liquid crystal element 31, the long axes of the liquid crystal molecules are aligned in the direction of +45 degrees or −45 degrees (homogeneous alignment). The light passed through the liquid crystal element 31 emerges as elliptically polarized light, whose ellipticity varies with the voltage applied to the liquid crystal element 31.
The light passed through the liquid crystal element 31 is split by a beam splitter 24 into reflected light and rectilinearly propagating light. The rectilinearly propagating light enters a quarter-wave plate 26A whose axis is oriented in the vertical axis direction, and the light is thus converted to linearly polarized light. At this time, as the polarization direction of the linearly polarized light depends on the ellipticity of the light passed through the liquid crystal element 31, the polarization direction varies depending on the voltage applied to the liquid crystal element 31. In this way, the polarization direction of the linearly polarized light can be modulated by the liquid crystal element 31. When the linearly polarized light whose polarization direction is thus modulated enters the test sample, the polarization direction is rotated by an unknown amount in accordance with the optical activity of the sample. The light passed through the sample enters a quarter-wave plate 26B where it is converted back to elliptically polarized light, and the elliptically polarized light enters an analyzer 27A. Of the components of the incident light, only the component vibrating in the same direction as the transmission axis of the analyzer 27A is passed through the analyzer 27A. The light passed through the analyzer 27A falls on a photodetector 29A where the light is converted into an electrical signal.
The reflected light separated by the beam splitter 24 is not directed toward the sample but is directed to an analyzer 27B. The light passed through the analyzer 27B falls on a photodetector 29B where the light is converted into an electrical signal.
The difference between the output signal of the photodetector 29A and the output signal of the photodetector 29B corresponds to the difference between the elliptically polarized light before entering the analyzer 27A and the elliptically polarized light before entering the analyzer 27B (that is, the angle of optical rotation through the sample). Accordingly, the angle of optical rotation through the sample can be measured from the difference between the output signal of the photodetector 29A and the output signal of the photodetector 29B, and the concentration of component in the sample can be determined from the angle of optical rotation through the sample.
While the angle of optical rotation can be obtained by the above method, actual test samples often contain optically active components other than the optically active component of interest. For example, when measuring sugar in urine (glucose in urine), the urine may contain vitamin C (specific rotation: 23°) due, for example, to an intake of a nutritional supplement. Since the molecular weight of glucose (180) is close to that of vitamin C (176), it is difficult to separate one from the other by the size of molecules.
A method that uses test strips is commonly employed as a method for analyzing urine. This method uses test strips coated with reagents for testing various components; that is, the test strips are immersed in the urine collected in a paper cup or the like, and the components of the urine are analyzed by the reagents that change color through chemical reactions. The change of color is checked by visual inspection or by using an optical sensor.
Further, there is also a method that uses an enzyme electrode method for the measurement of urine sugar; in this method, glucose in the urine is caused to undergo a chemical reaction by glucose oxidase (GOP), and the urine sugar level is quantified by measuring the generated current.
Patent document 1: Japanese Unexamined Patent Publication No. H09-145605 (FIG. 1)
Patent document 2: Japanese Unexamined Patent Publication No. H07-218889 (FIG. 3)
Patent document 3: Japanese Unexamined Patent Publication No. 2002-277387 (FIG. 3)
Patent document 4: Japanese Unexamined Patent Publication No. 2001-356089 (FIG. 3)